Time-reversed photoacoustic system and uses thereof

ABSTRACT

A remote mass is excited with one or more beams, and the surface vibrations of the excited mass are detected with one or more laser vibrometers. Each vibrometer generates a signal indicative of the surface vibrations which is stored, reversed in time, and applied to modulate an exciter beam that is then impinged onto the mass.

BACKGROUND

This application relates to methods and devices for remote internalinspection and ablation/modification of an object disposed within amass.

Methods for internal inspection of objects are known. Modern approachestypically use sonar methods for probing and/or altering the internalcomposition of a mass by applying acoustic waves to the mass, measuringthe vibrations induced in the excited mass such as by detectingscattered acoustic waves, and modifying subsequently applied acousticwaves in accordance with the scattered waves. For instance, U.S. Pat.No. 5,092,336 to Fink describes a method for focusing acoustic waves ona target in tissue wherein the tissue containing the target isilluminated with an unfocused acoustic beam, echo signals received by anarray of electro-acoustic transducers are stored, the distribution intime and the shapes of the echo signals are reversed, and the reversedsignals are applied to the transducers in the array to illuminate thetissue.

In U.S. Pat. No. 6,490,469 to Candy, the use of time reversed echosignals is expanded to a method for decomposing a plurality ofscatterers in tissue by iteratively transmitting a time reversed fieldinto the plurality of scatterers of the medium and performing a sequenceof time-reversal iterations to extract contribution of the i-thscatterer of the plurality of scatterers, estimating a weightingcoefficient of the i-th scatterer of the plurality of scatterers, andestimating the plurality of scatterers of the medium with the i-thscatterer removed, until a decomposition condition has been satisfied.

These methods and devices, and others like them, have met with practicalsuccess and have been applied to a variety of uses. However, they arelimited by their use of acoustic transducers for exciting the target andsurrounding mass and for applying the reversed signals to the mass, aswell as for measuring the vibrations of the excited mass. What is stillneeded are methods and devices for remotely focusing energy on a mass tolocate and/or destroy or otherwise alter targets disposed therein. Thepresent embodiments answer this and other needs.

SUMMARY

In a first embodiment disclosed herein, a system comprises an exciterdisposed to impinge at least one exciter beam onto a remote mass toexcite the mass, an optical probe disposed to impinge at least oneoptical beam onto a vibrating surface of the excited mass to bereflected thereby, a laser vibrometer disposed to detect at least partof the optical beam reflected by the vibrating surface of the excitedmass and configured to generate signals indicative of the surfacevibrations, a processor configured to store and reverse the signalsgenerated by the laser vibrometer, and a modulator configured tomodulate the exciter beam generated by the exciter in accordance withthe reversed signals.

In another embodiment disclosed herein, a system comprises a first lasersource disposed to impinge at least one first optical beam onto a remotemass to excite the mass, a second laser source disposed to impinge atleast one second optical beam onto a vibrating surface of the excitedmass to be reflected thereby, a laser vibrometer disposed to detect oneor more speckles from the second optical beam reflected by the vibratingsurface of the excited mass and configured to generate signalsindicative of the surface vibrations, a processor configured to storeand reverse the signals generated by the laser vibrometer, and amodulator configured to modulate the first beam generated by the firstlaser source in accordance with the reversed signals.

In a further embodiment disclosed herein, a time reversal mirrorcomprises a laser vibrometer disposed to detect one or more specklesfrom an optical beam reflected by a vibrating surface of a remoteexcited mass and configured to generate signals indicative of thesurface vibrations, a processor configured to store and reverse thesignals generated by the laser vibrometer, an exciter disposed toimpinge an exciter beam onto the remote mass, and a modulator configuredto modulate the exciter beam in accordance with the reversed signals.

In a still further embodiment disclosed herein, a method comprisesselecting a laser vibrometer configured to generate signals indicativeof detected optical beams, disposing the laser vibrometer to detect oneor more speckles from an optical beam reflected by a vibrating surfaceof a remote excited mass and to generate signals indicative of thesurface vibrations, providing the signals generated by the laservibrometer to a processor to store and reverse the signals, generatingan exciter beam to impinge onto the remote mass to excite the mass, andmodulating the exciter beam in accordance with the reversed signals.

These and other features and advantages of this disclosure will becomefurther apparent from the detailed description and accompanying figuresthat follow. In the figures and description, numerals indicate thevarious features of the disclosure, like numerals referring to likefeatures throughout both the drawings and the description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a functional diagram of a system as disclosed herein;

FIG. 2 is a flowchart of a method of use of the system of FIG. 1; and

FIG. 3 is a functional diagram of another system as disclosed herein.

DETAILED DESCRIPTION

Referring to FIG. 1, a system 100 in accordance with an embodimentdisclosed herein includes an exciter 110 for emitting one or moreexciter beams 112 towards a remotely located mass 114 that has a target116 disposed therein. The system 100 also includes an optical probe 120disposed to emit one or more optical beams 122 towards the mass 114. Alaser vibrometer 130 is further disposed to detect optical beamsreflected by the mass 114 such as speckles 124. The laser vibrometer isconnected to a processor 140 to communicate data thereto. The processoris further connected to control a modulator 150 that is configured tomodulate the exciter beam(s) 112 generated by the exciter 110.

With reference now to FIG. 1 and FIG. 2, in one method of use 200 ofsystem 100, exciter 110 is operated to generate 210 one or more exciterbeams 112 to impinge onto the mass 114 and thereby remotely excite themass. The exciter 110 may comprise any devices known to those skilled inthe art or heretofore undeveloped that may remotely excite an object ormass by any mechanism, including but not limited to mechanical, optical,or electrical. Thus, in one embodiment, the exciter 110 may emit 210 oneor more acoustic beams 112 to impinge upon the mass 114. In anotherembodiment, the exciter 110 may include a laser source for generating210 one or more laser beams 112 to impinge upon the mass 114. Any otherdevice and mechanism of remotely exciting the mass 114 may be usedwithin the spirit and scope of the present disclosure, and is onlylimited by the requirement that the beam(s) 112 emitted by the exciter110 must be capable of being amplitude and phase modulated. In a typicalembodiment, the exciter 110 may include a pulsed laser source togenerate 210 one or more pulsed laser exciter beams 112.

With continued reference to FIGS. 1 and 2, upon impinging onto the mass114, the exciter beam(s) 112 generates an acoustic impulse 115 thattravels through the mass and impinges unto the target 116 disposedtherein. The acoustic excitation mechanism that gives rise to theresultant acoustic impulse 115 traveling through the mass 114 is not ofimportance to the practice of the disclosure (e.g., it may bethermoelastic, ablative, or otherwise). The resultant acoustic impulse115 thus generated may be either a surface wave (examples of such wavesinclude Rayleigh and Lamb waves), or may be a compressional wave thatsamples the surface or the volume of the mass 114, respectively. Theresultant acoustic impulse 115 traveling through the mass 114 willeventually strike and scatter from the buried target 116. Uponscattering from the buried target 116 or after becoming dispersivelyspread upon propagation along the surface of the mass 114, the resultantacoustic impulse will cause the surface of the mass (e.g., the ground orthe external surfaces of a sample) to vibrate due to the initial pulse115, harmonics (e.g., due to nonlinearities), and any acoustic echoes117 from the target 116.

For purposes of discussion only, optical probe 120 is shown disposedabove the mass 114 to emit 220 and impinge one or more optical (e.g.,laser) beams 122 onto the mass at approximately the same point of impactas the exciter beam(s) 112. The optical probe beam(s) 122 reflect offthe surface of the mass 114 as multiple beams 124. Because the surfaceof the mass 114 is vibrating, the reflected beams 124 will allexperience a corresponding Doppler shift. A portion of the reflectedbeams 124 may be bounced off a reflecting mirror 170 into a beamclean-up optical element 160 to sum 230 the reflected beams into asingle optical beam 164 having the same Doppler shift as the reflectedbeams. A variety of beam clean-up optics are known and available tothose skilled in the art, and the particular method or device usedwithin the context of the present disclosure is not important to thescope of the embodiments disclosed herein. It is also expressly notedthat certain, currently available laser vibrometers include both theoptical probe beam generator (e.g., a laser) as well as the speckledetector and beam clean-up optics into a single, unitary device. Thepresent embodiment separates these elements for ease of discussion andclarity of disclosure.

The single, “cleaned” optical beam 164 is provided to the laservibrometer 130 to sense or detect 240 the phase shift of the beam andthus the vibrations and/or displacement speed of the surface of the mass114. A laser vibrometer typically operates on the Doppler principle tocompare the wavelength or frequency of the optical probe beam(s) 122with that of the reflected beams 124, such as by the use of aninterferometer. The difference between the two wavelengths orfrequencies is indicative of the amplitude and frequency of the surfacevibrations under investigation. The laser vibrometer 130 thus generates250 a signal 132 indicative of the surface vibrations of the mass 114.The signal 132 is in essence a series of pulses in time indicative ofthe amplitude and frequency of vibration of the surface of the mass 114.It is noted that a compensated vibrometer may be employed as an adaptivephotodetection receiver and thus provide any desired beam clean-up. Anadaptive photodetector provides phasing of all the detected speckles,thereby enhancing the signal-to-noise (or, equivalently, the surfacedisplacement sensitivity) of the system 100.

The signal 132 generated by the laser vibrometer 130 is provided to aprocessor 140 for storing and temporal reversing 260. The processor 140may be equipped with a cache memory to effectively store the signal 132in a software equivalent of a programmable delay line network. Theprocessor is further programmed to reverse the impulses of the signal132 in time and thus to output a time-reversed signal 142. Thus, afterthe entire pulse stream is stored in the cache memory, the delay linethen outputs the pulsetrain of the signal 132 in a reverse temporalsequence 142, so that the last feature into the given delay line emergesas the first feature out from the given delay line.

The time-reversed signal 142 is applied to the modulator 150, which isconnected to the exciter 110 to modulate 270 the exciter beam(s) 112emitted thereby in accordance with the time-reversed signal 142. Thus,after initially exciting the mass 114 with an unfocused beam or beams112, the exciter 110 proceeds to apply time-reversed signals to the samearea of the mass 114 as the initial exciter beam. In this manner, thetime-reversed signal 142 is a time-reversed acoustic replica of thedetected signal 132, and it will be generated into the mass 114 at thesame physical location as was the respective received acousticinformation 117. As known to those skilled in the art, applying such atime-reversed acoustic replica signal will concentrate energy deliveredby the time-reversed signal 142 at the target 116, thereby optimizingthe performance and signal-to-noise of the system 100 for mapping theinternal structure of the mass 114, or locally modifying and/or ablatingthe target 116. The process of detecting the vibrations of the surfaceof the mass 114, reversing the vibration pulses and applying thetime-reversed pulse train 112 to the mass can be repeated as many timesas desired to achieve the desired result of imaging ormodifying/ablating the target 116. As will be appreciated, thecombination of the laser vibrometer 130, processor 140, modulator 150and exciter 110 essentially forms a time reversal mirror for reversingthe surface vibrations of the excited mass 114. In a further embodiment,the modulator 150 may modulate the exciter beam(s) 112 with a temporallysampled reversed signal in accordance with the Nyquist sampling theorem.

System 100 and its method of use as outlined above can thus be appliedto a wide variety of uses, including but not limited to, detection ofburied structures in terrestrial and ocean-based applications (e.g.,explosive mines), remotely mapping objects behind opaque media such asburied objects in walls, remote sensing of material attributes,nondestructive evaluation of engines and special coatings, detection andmapping of defects in epoxy bonds, spot welds and other bonds, enhancedsensitivity for composite material evaluation, detection of undesirabledefects and inclusions in metallic and ceramic media, medical proceduressuch as kidney or gall stone ablation and cauterizing, etc. The methodand system and for in-situ manufacturing process control applicationsfor improved yield and quality assurance, etc. The target 116 can be anobject that is of different composition than the surrounding mass 114,or may be a defect or aberration such as an occlusion or a crack withinthe mass.

The system 100 may also be formed with a plurality of vibrometersarranged in an array, such as a phased-array receiver configuration, todetect the surface vibrations of the mass 114. In such a configuration,the processor must be equipped with a separate, parallel delay line foreach vibrometer to store and time-reverse the signal received from eachvibrometer. An equal number of exciters are then each modulated with arespective time-reversed signal to impinge exciter beams onto the mass114.

With reference now to FIG. 3, another embodiment in accordance with thepresent disclosure may comprise an array 300 of identical systems ormodules 100, 100′, 100″, . . . , each module assembled as describedpreviously. Such an array 300 can provide enhanced imaging performanceby virtue of its ability to address a different location on the surfaceof the mass 114 to be interrogated, thereby generating and acquiringadditional information regarding the size, shape, and acoustic (elastic)properties of the target 116. In addition, an array 300 using multiplemodules may detect targets 116 that would otherwise be obscured by othernatural (rocks, tree roots, etc.) or man-made objects in the path of theacoustic impulses 115, 115′, 115″, . . . , generated by the exciterbeam(s) of each of the modules 100, 100′, 100″, . . .

With continued reference to FIG. 3, each of the modules 100, 100′, 100″,. . . may be assembled to contain a complete time-reversal subsystemhaving the capability of probing the surface of the mass 114 at a point(or array of points) using a laser vibrometer and beam clean-upapparatus (to compensate for speckle and relative platform motion, etc),along with a cache memory/processor to store and readout the surfacevibration data, and a laser exciter (or array of exciters) to induceacoustic waves into the mass 114 with a beam 122 modulated in accordancewith the time-inverted signal 132, all as previously described herein.The system 300 may further contain a central processor 310 transmittingand receiving information 312 to/from each module 100, 100′, 100″, . . ., including information regarding the surface vibrations detected byeach module as well as control data for controlling the operation ofeach module. The central processor 310 may also provide data 314regarding the detected target 116 for further analysis and imaging.Furthermore, an auxiliary laser exciter 320 may also be provided forexciting the mass 114.

An array 300 as described above may be operated in several differentmodes which may be selected based on the nature of the target 116 to bedetected relative to the type of mass 114 in which it is immersed (e.g.,multiple layered structures with hidden features such as defects incomposite materials, embedded undesirable objects, such as rocks underground, etc.). In one possible mode of operation, the laser exciter ofone time-reversal module 100′ (or auxiliary module) may be designated asthe “master exciter” to induce acoustic impulses 115′ in the mass 114that scatter from the target 116 as acoustic echoes 117′ and inducesurface vibrations in the mass. The resultant acoustic informationdetected by the other time-reversed modules 100, 100″ then drives therespective excitation lasers (the “slave exciters”) of each such module,each exciter beam being modulated by its respective time-reversedsignal. All the information may then processed by the central processor310 for imaging and analysis of the target 116.

In another mode of operation, the laser exciter of each module 100,100′, 100″, . . . may act as the master exciter sequentially, to induceacoustic impulses 115, 115′, 115″ respectively in the mass 114 thatscatter from the target 116 as acoustic echoes 117, 117′, 117″respectively and induce surface vibrations in the mass. The data fromall the modules may be processed centrally by the central processor 310.The central processor 310 may also operate to designate each modulesequentially as the master and control the overall process. The order orsequence in which the modules are designated as the master-exciter canbe random or predetermined. This mode of operation enables gatheringinformation from the entire ensemble of time-reversal modules and isalso more robust because the target 116 is acoustically probed from aplurality of different directions. This mode of operation istheoretically similar to a CAT scan and may be employed withconventional image processing software and algorithms to provide a 3-Dreconstruction of the target as well as its elastic properties, whichmay not be homogeneous or uniform.

We note that as opposed to existing ultrasound imaging array systems(which require direct contact, immersion into water tanks, liquid-spraycontact, etc), the present disclosure enables the interrogation to berealized without physical contact to the object under interrogation. Infact, when employing thermoelastic (as opposed to ablative) excitationmodes or laser-induced plasmas above the surface, the object can, inprinciple, be examined without any cosmetic or material damage inducedby the laser beams, enabling truly nondestructive testing to berealized. In addition, the laser system enables robust interrogation andexcitation to be realized, in that the need for precise alignment ofcontact transducers to the object (e.g., normal incidence) is relaxed bythe laser-based system. That is, a laser beam can be inclined atrelatively large angles to the surface under test, and still result inacoustic modes that propagate normal to the surface (as well as surfacewaves, etc. if needed).

In further embodiments, each time-reversal module 100 may operate itsrespective exciter 110 and probe 120 at different wavelengths if the twoutilize separate laser sources. For purposes of example only, in oneembodiment, the wavelength of the laser probe 120 may be chosen toreflect or scatter from the surface of the mass 114 with the greatestoptical reflectivity, while the wavelength of the exciter 110 may bechosen to most effectively induce acoustic impulses 115 in the massusing a wavelength with the greatest optical absorption by the mass. Inother embodiments, the wavelength of both the laser probe and theexciter can be identical, whereas in other embodiments the same lasersource can be used for both probing and exciting the object.

Having now described the invention in accordance with the requirementsof the patent statutes, those skilled in this art will understand how tomake changes and modifications to the present invention to meet theirspecific requirements or conditions. Such changes and modifications maybe made without departing from the scope and spirit of the invention asdisclosed herein.

1. A system, comprising: an exciter disposed to impinge at least oneexciter beam onto a remote mass to excite the mass; an optical probedisposed to impinge at least one optical beam onto a vibrating surfaceof the excited mass to be reflected thereby; a compensated laservibrometer disposed to detect at least part of the optical beamreflected by the vibrating surface of the excited mass and configured togenerate signals indicative of the surface vibrations, the vibrometerincluding an adaptive photodetector for detecting a plurality ofspeckles from the optical beam reflected by the vibrating surface of theexcited mass; a processor configured to store and reverse the signalsgenerated by the laser vibrometer; and a modulator configured tomodulate the at least one exciter beam generated by the exciter inaccordance with the reversed signals.
 2. The system of claim 1, whereinthe exciter is selected from the group of exciters comprised of lasersources configured to impinge at least one laser exciter beam onto theremote mass to excite the mass, and acoustic sources configured toimpinge at least one acoustic exciter beam onto the remote mass toexcite the mass.
 3. The system of claim 2, wherein the processor isconfigured to store the signals as a series of pulses and to reverse thestored pulses in a first in, last out (FILO) sequence.
 4. The system ofclaim 3, wherein the processor comprises: a cache memory to store thesignals.
 5. The system of claim 3, wherein the processor comprises: aprogrammable delay line network.
 6. The system of claim 5, wherein theprocessor comprises: a cache memory to store the signals.
 7. The systemof claim 2, wherein the exciter is a pulsed laser source for impingingan optical beam onto the remote mass to excite the mass.
 8. The systemof claim 2, further comprising: a plurality of laser vibrometersdisposed to detect a plurality of speckles from the optical beamreflected by the vibrating surface of the excited mass and configured togenerate signals indicative of the surface vibrations.
 9. The system ofclaim 8, wherein the plurality of laser vibrometers are disposed in apredetermined array.
 10. The system of claim 9, wherein thepredetermined array of laser vibrometers is a phased array.
 11. Thesystem of claim 9, wherein the processor comprises: a plurality ofprogrammable delay line networks, each configured to store and reversethe signals generated by a respective one of the plurality ofvibrometers.
 12. A system, comprising: a first laser source disposed toimpinge at least one first optical beam onto a remote mass to excite themass; a second laser source disposed to impinge at least one secondoptical beam onto a vibrating surface of the excited mass to bereflected thereby; a compensated laser vibrometer with an adaptivephotodetector disposed to detect one or more speckles from the secondoptical beam reflected by the vibrating surface of the excited mass andconfigured to generate signals indicative of the surface vibrations; aprocessor configured to store and reverse the signals generated by thelaser vibrometer; and a modulator configured to modulate the at leastone first beam generated by the first laser source in accordance withthe reversed signals.
 13. The system of claim 12, wherein the processoris configured to store the signals as a series of pulses and to reversethe stored pulses in a first in, last out (FILO) sequence.
 14. Thesystem of claim 13, wherein the processor comprises: a cache memory tostore the signals.
 15. The system of claim 13, wherein the processorcomprises: a programmable delay line network.
 16. The system of claim15, wherein the processor comprises: a cache memory to store thesignals.
 17. The system of claim 12, wherein the first laser source is apulsed laser source.
 18. The system of claim 12, further comprising:beam clean-up optics disposed to combine one or more speckles from thesecond optical beam reflected by the vibrating surface of the excitedmass into a single coherent beam for detection by the laser vibrometer.19. The system of claim 12, further comprising: a plurality of laservibrometers disposed to detect a plurality of speckles from the beamreflected by the vibrating surface of the excited mass and configured togenerate signals indicative of the surface vibrations.
 20. The system ofclaim 19, wherein the plurality of laser vibrometers are disposed in apredetermined array.
 21. The system of claim 20, wherein thepredetermined array of laser vibrometers is a phased array.
 22. Thesystem of claim 20, wherein the processor comprises: a plurality ofprogrammable delay line networks, each configured to store and reversethe signals generated by a respective one of the plurality ofvibrometers.
 23. A time reversal mirror, comprising: a compensated laservibrometer with an adaptive photodetector disposed to detect one or morespeckles from an optical beam reflected by a vibrating surface of aremote excited mass and configured to generate signals indicative of thesurface vibrations; a processor configured to store and reverse thesignals generated by the laser vibrometer; an exciter disposed toimpinge an exciter beam onto the remote mass; and a modulator configuredto modulate the exciter beam in accordance with the reversed signals.24. The time reversal mirror of claim 23, wherein the exciter isselected from the group of exciters comprised of laser sourcesconfigured to impinge at least one laser exciter beam onto the remotemass, and acoustic sources configured to impinge at least one acousticexciter beam onto the remote mass.
 25. The time reversal mirror of claim24, wherein the processor is configured to store the signals as a seriesof pulses and to reverse the stored pulses in a first in, last out(FILO) sequence.
 26. The time reversal mirror of claim 25, wherein theprocessor comprises: a cache memory for storing the signals.
 27. Thetime reversal mirror of claim 25, wherein the processor comprises: aprogrammable delay line network.
 28. The time reversal mirror of claim27, wherein the processor comprises: a cache memory for storing thesignals.
 29. The time reversal mirror of claim 23, wherein the exciteris a pulsed laser source for impinging an optical beam onto the remotemass to excite the mass.
 30. The time reversal mirror of claim 23,further comprising: a plurality of laser vibrometers disposed to detecta plurality of speckles from the optical beam reflected by the vibratingsurface of the excited mass and configured to generate signalsindicative of the surface vibrations.
 31. The time reversal mirror ofclaim 30, wherein the plurality of laser vibrometers are disposed in apredetermined array.
 32. The time reversal mirror of claim 31, whereinthe predetermined array of laser vibrometers is a phased array.
 33. Thetime reversal mirror of claim 31, wherein the processor comprises: aplurality of programmable delay line networks, each configured to storeand reverse the signals generated by a respective one of the pluralityof vibrometers.
 34. A method, comprising: selecting a compensated laservibrometer configured to generate signals indicative of detected opticalbeams and including an adaptive photodetector; disposing the laservibrometer for the adaptive photodetector to detect one or more specklesfrom an optical beam reflected by a vibrating surface of a remoteexcited mass and to generate signals indicative of the surfacevibrations; providing the signals generated by the laser vibrometer to aprocessor to store and reverse the signals; generating an exciter beamto impinge onto the remote mass to excite the mass; and modulating theexciter beam in accordance with the reversed signals.
 35. The method ofclaim 34, wherein generating the exciter beam comprises: generating anexciter beam selected from the group comprised of laser beams andacoustic beams.
 36. The method of claim 35, wherein providing thesignals to the processor comprises: providing the signals to theprocessor to store the signals as a series of pulses and to reverse thestored pulses in a first in, last out (FILO) sequence.
 37. The method ofclaim 35, wherein providing the signals to the processor comprises:providing the signals to the processor to store and reverse the signalsin a programmable delay line network.
 38. The method of claim 35,wherein generating the exciter beam comprises: generating a pulsed laserbeam.
 39. The method of claim 35, further comprising: disposing aplurality of laser vibrometers to detect one or more speckles from theoptical beam reflected by the vibrating surface of the remote excitedmass, each laser vibrometer configured to generate signals indicative ofthe surface vibrations.
 40. The method of claim 39, wherein disposingthe plurality of laser vibrometers comprises: disposing the plurality oflaser vibrometers in a predetermined array.
 41. The method of claim 40,wherein disposing the plurality of laser vibrometers comprises:disposing the plurality of laser vibrometers in a phased array.
 42. Themethod of claim 40, wherein providing the signals to the processorcomprises: providing the signals to the processor to store and reversethe signals generated by each one of the plurality of vibrometers in arespective one of a plurality of programmable delay line networks.